Abstract:

System for the active and passive stabilization of a vessel (10), such as
ships, boats, rigs, barges, platforms and cranes operating in a maritime
environment, which vessel (10) is provided with tanks (Ha-d) to provide
buoyancy and/or ballast, which tanks (11a-d) are provided with openings
(12a-d) in the bottom, which openings (12a-d) are facing the medium in
which the vessel (10) is floating. The tanks (11a-d) are independent of
each other and the openings (12a-d) are so large that a sufficient volume
of fluid can pass without cavitation or other resistance, and the system
includes means (13a-d) for supplying fluid to the tanks (11a-d),
controlled to counteract the effects of external forces on the movements
of the vessel (11). The invention further includes methods for the
passive and active stabilization of the vessel by use of the system.

Claims:

1. System for the active and passive stabilization of a vessel (10), such
as ships, boats, rigs, barges, platforms and cranes operating in a
maritime environment, which vessel (10) is provided with one or more
tanks (11a-d) to provide buoyancy and/or ballast, which tanks (11a-d) are
provided with openings (12a-d) in the bottom, which openings (12a-d) are
facing the medium the vessel (10) is floating in, which tanks (11a-d) are
independent of each other,characterized in that:the one or more tanks
(11a-d) are arranged in such a way that they have an extension so that at
least a substantial part of the tanks (11a-d) extends above a fluid level
the vessel (10) is floating in, and at least a part of the tanks (11a-d)
extends below the fluid level the vessel (10) is floating in, as the
vessel (10) is floating in normal conditions without any loads,the system
includes means (13a-d) for providing positive or negative pressure in the
tanks (11a-d) for the removal or supply of fluid, respectively, by
directly adding positive or negative pressure to the interior of the
tanks (11a-d) and which positive or negative pressure is acting directly
on the surface of the fluid in the tanks (11a-d), which means (13a-d) are
controlled to counteract the effects of external forces on the movements
of the vessel (10).

2. System according to claim 1, characterized in that the system further
includes means, such as pressure sensors/meters, floats, pressure pulses
and/or similar, to provide information on the state in the tanks (11a-d).

3. System according to claim 1, characterized in that the system further
includes means for finding information on the movements of the vessel,
such as a MRU (Motion Reference Unit) and/or a VRU (Vertical Reference
Unit) and/or a dynamic positioning system or similar means, which
provides information on the movements of the vessel (10), mainly the
vertical movements.

4. System according to claim 1, characterized in that the system further
includes sensing means (14), such as pressure sensors and/or radar and/or
laser and/or wave calibration or similar means, which means (14)
preferably are arranged along the sides of the vessel to provide
information on wave height and frequency.

5. System according to claim 3, characterized in that the system includes
means for predicting the movements of the vessel based on information on
the movements of the vessel and/or the sensing means (14) in order to
counteract the wave movements before the vessel is affected by the wave.

6. System according to claim 1, characterized in that the system further
includes a control system to control the fluid volume in the tanks
(11a-d), by providing negative pressure in the thanks (11a-d) for ballast
or positive pressure for buoyancy.

7. System according to claim 1, characterized in that the means (13a-d)
for supplying fluid to the tanks (11a-d) are vacuum/pressure compressor
and/or valves, where at least one of these means (13a-d) is arranged to
each of the tanks (11a-d).

8. System according to claim 1, characterized in that the tanks (11a-d)
are adapted to the vessel (10) as regards size and shape to available
space in the vessel (10), and are arranged close to the front, rear
and/or middle parts of the vessel (10) to provide the vessel (10) with
the desired properties.

9. System according to claim 6, characterized in that the control system
is arranged to calculate current ballast and/or buoyancy for the
different tanks (11a-d), based on input from means for finding/predicting
the movements of the vessel, and/or means for information on the state in
the tanks (11a-d), and/or sensor means (14) for information on wave
height and frequency, and/or given predefined parameters for the
behaviour of the vessel, and provide means (13a-d) for supplying fluid to
the tanks (11a-d) with settings.

10. System according to claim 1, characterized in that the system is
manual or automatic.

11. System according to claim 1, characterized in that the system further
includes means, such as valves, throttles, specially formed propellers
(20, 21) or similar, arranged to the openings (12a-d) of the tanks
(11a-d), to close the openings and/or supply fluid to the tanks (11a-d).

12. System according to claims 7, characterized in that the free capacity
of the vacuum/pressure compressors (13a-d) is used to:provide bottom
tanks of a vessel with air supply, ortransport cooling water from a
vessel's sea chest and via the coolers of the vessel, orevaporate water
from contaminated bilge water and expel pure water vapour out in the
atmosphere.

13. Method for active stabilization of a vessel (10), which vessel (10) is
provided with a system according to claim 1, characterized in that it
includes the following steps;1. Acquiring information on the movements of
the vessel,2. Acquiring information on the state in the tanks of the
system,3. Based on information from the steps 1 and 2, calculating the
ratio of filling for the tanks, i.e. if the pressure in the tank is to be
positive or negative,4. Providing means for controlling the pressure in
the tanks with settings based on the calculation in step 3,5. Increasing
or decreasing the pressure in the tanks by means of means for controlling
the fluid volume in the tanks, until means for the state in the tanks
respond to the control system according to the invention that the desired
positive or negative pressure is achieved,6. Repeating the steps 1-5.

14. Method according to claim 13, characterized in that step 1 includes
acquiring information from a MRU (Motion Reference Unit) and/or a VRU
(Vertical Reference Unit) and/or a DP system or similar, which
information includes information on the movements of the vessel.

15. Method according to claim 13, characterized in that step 2 includes
acquiring information on the state in the tanks by suitable means for
this, such as pressure sensors/meter, floats, pressure pulses or similar,
which is a premise for the control system according to the invention to
know if pressure or vacuum is to be supplied to the tanks.

16. Method according to claim 13, characterized in that step 1 and/or 2
also includes acquiring information on wave height and frequency by means
of sensor means for this, which information makes it possible for the
control system to form a picture of the wave frequency, direction of the
wave and the total changing buoyancy provided by the wave.

17. Method according to claim 13, characterized in that step 4 and 5
include providing the means for controlling the fluid amount in the tanks
with settings to achieve desired ballast or buoyancy in the tanks.

18. Method according to claim 17, characterized in that the pressure in
the tanks is increased or decreased until means for the state in the
tanks respond to the control system that the desired pressure is
achieved.

19. Method according to claim 13, characterized in that the steps 1 to 5
are continuously repeated for the vessel to adapt to the continuously
changing environment, which makes the system self correcting.

20. Method for passive stabilization of a vessel, which vessel is provided
with the invention according to claim 1, characterized in that it
includes the following steps:1. Acquiring information on the movements of
the vessel,2. Acquiring information on the state in the tanks of the
system,3. Based on information from the steps 1 and 2, calculating if the
tanks should have reduced and/or increased buoyancy,4. Providing means
for controlling the fluid amount in the tanks with settings to open a
valve for supplying fluid to the tanks if reduced buoyancy in tanks is
required and/or to close the valve if increased buoyancy in the tanks are
required,5. Continuously repeating the steps 1-4.

21. Method according to claim 20, characterized in that step 1 includes
acquiring information from a MRU (Motion Reference Unit) and/or a VRU
(Vertical Reference Unit) and/or a DP system or similar, which
information includes information on the movements of the vessel.

22. Method according to claim 20, characterized in that step 2 includes
acquiring information on the status of the tanks by suitable means for
this, such as pressure sensors/meters, floats, pressure pulses or
similar, which is a premise for the control system according to the
invention to know if positive or negative pressure is to be provided in
the tanks.

23. Method according to claim 20, characterized in that step 1 and/or 2
also include acquiring information on wave height and frequency by means
of sensor means for this, which information makes it possible for the
control system to form a picture of the wave frequency, direction of the
wave and the total changing buoyancy provided by the wave.

24. Method for passive stabilization of a vessel according to claim 19,
characterized in that if no information from step 1 and 2 is present, the
valve can be manually adjusted for best possible effect by trial and
experience.

Description:

[0001]The invention relates to a system for the active and passive
stabilization of a vessel according to the preamble of claim 1. The
invention also relates to a method for the active and passive
stabilization of a vessel by means of the system described in claims 12
and 19.

BACKGROUND

[0002]Today, most vessels are not provided with active stabilization,
although for vessels working together with fixed installations this is
desired, and this has until now been considered as a natural thing and
taken as a matter of course. Where vessels do not have this feature, they
must, in bad weather and unfavourable wave conditions, space themselves
at a distance, waiting for weather changes. Even when weather conditions
are relatively good and the motions of the vessel are small, they are
very vulnerable in relation to the wave movements and completely at their
mercy. Not only vessels, as ships or boats, but also rigs, platforms,
cranes etc., will benefit greatly from active stabilization. A vessel
without a suitable system for active stabilization may be compared to a
car without shock absorbers, which would be unthinkable in terms of road
safety.

[0003]Seagoing vessels are, as is known, affected by the movement of waves
and other static loads.

[0004]From prior patent literature, among other things, the use of tanks
which are open in the bottom, is known especially on oil rigs. These
tanks function by having an adjustable valve at their top part, which
leads into the atmosphere. Because of the static movement that the rig
experiences in high seas, the ratio of filling of sea water in the tanks
can be adjusted to compensate and reduce the movement.

[0005]Other systems which presently used to avoid roll are stabilization
tanks and Anti Heeling pumps, although in these cases a large proportion
of the loadbearing capacity of the vessel can not be used. The vessel is
in addition exposed to constant loading without any possibility of
controlling changes, for example, in the draught. The Anti Heeling pump
is an active pump system, but it often has insufficient capacity in
relation to what is desired.

[0006]In prior art systems such as this, it is a huge problem that, as
fluid is pumped from one full tank to another, making effectively two
tanks which are each half full with ballast with a free fluid surface. In
terms of stability, this is a dangerous situation. Another problem with
the prior art systems is that only one valve is used to close the flow of
fluid between the tanks, which results in an unstable system, especially
if the valve is locked in the open position, in which case the fluid in
these two tanks can flow freely between the two tanks, from starboard to
port.

[0007]From GB 2 091 192A a vessel is known, which is provided with tanks
for stabilization. These tanks have openings in the bottom and are used
for active and passive stabilization. A major disadvantage of GB 2 091
192 A is that only compressed air and storage tanks are used for active
and passive stabilization, with low pressure (% to % bar) and high
pressure (3 to 7 bar), which means that all changes of the level in the
ballast tanks must take place below the water level, and only the
buoyancy in the ballast tanks can be changed. This also means that a
limited ballast volume is available.

[0008]Vessels performing anchoring operations are often provided with
bulky machinery and have a high consumption of diesel fuel so that, a
planned operation may take much longer time than expected and will result
in reduced stability as the diesel tanks are gradually emptied.

[0009]Present vessels with stabilization tanks are vulnerable if a
critical situation arises, for example, a power failure or similar, in
which case the vessel may not be able to transfer ballast.

[0010]Where vessels are required to carry out towing operations, this is
done by changing direction by rudder deflection, with the result that the
load can be moved very quickly from starboard to port and with the
present systems it is not possible for the stabilization to be
transferred rapidly enough.

[0011]In connection with towing operations, a long tow rope with weights
is used between the towing vessel and the object being towed. This is to
reduce the variations in the tension of the tow rope due to constant
changes in the wave resistance. After each wave the vessel must
accelerate to recover the velocity the vessel had before it hit the wave.
The more this wave resistance can be reduced, the more economical it will
be for the planned operation.

[0012]It is generally very desirable for the influence on the environment
from vessels to be reduced, both in relation to economics, but also in
relation to NOx emissions. This will result in a better environment and
less pollution--an important topic in the community today.

[0013]The lack of active stability in relation to movement also affects
all vessels where a certain amount of manual work has to be carried out
by people. A fishing boat is an example of a kind of vessel where the
considerable damage can be caused as a result of the many manual tasks
performed during catching and processing of fish. The speed of
stabilization may differ between an accident and a normal operation. A
system which can more rapidly stabilize and compensate forces affecting
the vessel is highly desired.

[0014]Many vessels are designed to pass through certain lock systems for
waterways or shallow waters, and these vessels are designed to always
have a low draught, something which may result in stability problems.

[0015]Icebreakers are another kind of vessel, which have a special hull
design associated with the properties of breaking ice. These vessels must
carry large amounts of ballast, which has to be transferred from the
stern to the prow of the vessel and this can produce uncontrolled
situations. The movement of ballast will always have an uncontrolled
effect on all ships.

[0016]A number of vessels throughout the world lie on standby due to high
seas in case their attendance is required. This can result in unnecessary
pollution and unnecessary costs, as the vessels must lie idle on standby.
It is thus very desirable that a vessel should be able to operate during
worse conditions than is the case for present vessels, while at the same
time ensuring the safety of vessels and crews.

[0017]Also for vessels having a helicopter deck, there will be a need for
improved stability and compensation for vertical movements, as the
helicopter will not be able to land if the movements of the vessel are
too great.

[0018]It is therefore obvious that there is a need for most vessels to
have a more rapid and active stabilization of vessel than is the case
today. There is also presently a lack of passive stabilization of
vessels.

OBJECT

[0019]The main object of the invention is to provide a system and methods
for the active and passive stabilization of a vessel, especially
controlling the vertical movement of all floating vessels/barges and
rigs/platforms caused by the effect of waves, displacements or movement
of load/ballast and crane work.

[0020]It is further an object of the invention to reduce the maximum
movement which affects vessels today, i.e. reducing pitch, roll and
draught.

[0021]It is further an object of the invention that it should be possible
to maintain the distance between the vessel and the sea bottom as
constant as possible.

[0022]It is finally an object of the invention that the system and methods
should improve or entirely eliminate the above mentioned disadvantages of
prior art systems, and result in improved security for both crew and
vessels operating in maritime environments, which are provided with the
system and methods according to the invention.

THE INVENTION

[0023]A system according to the invention for the active and passive
stabilisation of vessels, such as ships, boats, rigs, barges, platforms
and cranes, operating in a maritime environment, is described in claim 1.
Preferable features of the system are described in claims 2-11.

[0024]A method according to the invention for the active stabilization of
vessels, such as ships, boats, rigs, barges, platforms and cranes,
operating in a maritime environment, by means of a system according to
the invention, is described in claim 12. Preferable features of the
method are described in claims 13-18.

[0025]A method according to the invention for the passive stabilization of
vessels, such as ships, boats, rigs, barges, platforms and cranes, which
operate in a maritime environment, by means of the system according to
the invention, is described in claim 19. Preferable features of the
method are described in claims 20-23.

[0026]All floating objects which are to be referred to, and which are to
be controlled according to the invention, will hereinafter be referred to
as vessels.

[0027]A system according to the invention mainly includes tanks, means for
supplying and removing fluid to and from the tanks, and a control system
for controlling the means for supplying and removing fluid to and from
the tanks, based on information on the movements of the vessel and the
effects of the environment on the vessel. A vessel to make use of the
system and the methods according to the invention is advantageously
provided/designed with adapted tanks on adapted locations, having an
opening at the bottom, which is large enough for a sufficient volume of
fluid to pass without cavitation or other resistance in the openings of
the tanks.

[0028]The tanks preferably further are of a sufficient height in relation
to the sea level, such that a sufficient volume of fluid can compensate
the buoyancy which produces changes in pitch, roll and draught of the
vessel. At the upper part of the tanks, there are arranged means for
supplying and removing fluid to and from the tanks, for example, vacuum
compressors or similar, which are used to control the pressure/vacuum
over the fluid surface in the tanks, and in this way can raise the fluid
level in the tanks to provide the desired ballast or lower the fluid
level to provide buoyancy for the vessel at any time. The fluid volume in
the tanks is controlled by the control system so that the fluid level is
changed to compensate the forces affecting the vessel, such as the
movement of the sea on the vessel or other components/loads affecting the
vessel, which results in vertical movements.

[0029]In a traditional vessel, the roll, pitch and draught movements are
usually compensated by means of the displacement of floating fluid in
tanks, which fluid amounts are a part of the tonnage of the vessel. As
described above, this is "closed" systems which may result in stability
problems, especially in cases of failure, as these systems will not
provide sufficient ballast/buoyancy within an acceptable time, due to
limitations in the available total fluid volume and capacity of pumps. In
addition they reduce the total load capacity of the vessel, as the fluid
volume is a part of the tonnage of the vessel.

[0030]By means of the invention, the tanks do not basically include any
fluid amount, but will be provided with fluid through the operation of
the system, and only as required. In this way, the vessel will have a
maximum load capacity. Because the system utilizes the medium in which it
floats to provide ballast for the vessel, this results in no limitations
in relation to fluid volume, as long as the tanks are appropriate for the
vessel and arranged at suitable locations of the vessel. As the tanks are
open against the medium the vessel is floating in, the vessel will be
able to utilize this entire medium as fluid supply.

[0031]The system according to the invention includes, as mentioned above,
a control system for the control of ballast/buoyancy in the tanks. The
control system will receive information from different sources on the
status of the tanks at any time, and information on the movements of the
vessel. Information on the movements of the vessel can, for example, in
one embodiment, be provided by a MRU (Motion reference unit) and VRU
(Vertical reference unit), which provide information on the vertical
movements of the vessel or similar, i.e. with roll, pitch and draught
references. Here it also will be advantageous with, for example, gyro
stabilization. In the offshore industry, most vessels are provided with a
DP system. DP--Dynamic Positioning--is basically a method for holding a
ship and semi-submersible rigs in the same horizontal position above the
sea bed maintaining the same direction or maintaining the same position
in relation to another vessel or floating structure without the use of an
anchor, by using the vessel's own propellers and thrusters. The DP system
includes means for predicting changes before they actually happen, to
compensate for changes in the environment around the vessel thereby
ensuring a steady operation. If a vessel is provided with a DP system,
the control system according to the invention can utilize the information
from this on the movements of the vessel.

[0032]A method for active stabilization according to the invention can be
summarized in the following steps: [0033]1. Acquiring information on the
movements of the vessel from a MRU and/or a VRU and/or a DP system or
similar, which provides information on the movements of the vessel,
[0034]2. Acquiring information on the state in the tanks of the system,
[0035]3. Based on information from the steps 1 and 2, calculating the
ratio of filling for the different tanks by means of a control system
according to the invention, i.e. if vacuum and/or pressure is to be
supplied, where pressure only is supplied if the level of the tank is to
be lower than the fluid level in which the vessel is floating, [0036]4.
Providing means for controlling vacuum and/or pressure in the tanks with
settings based on the calculation in step 3, [0037]5. Supplying pressure
and/or vacuum to the tanks by means of means for controlling the fluid
volume in the tanks, until the means for the status of the tanks respond
to the control system according to the invention that the desired
pressure and/or vacuum is achieved, [0038]6. Repeating the steps 1-5.

[0039]Steps 1 and/or 2 can in addition to acquiring information on the
movements of the vessel also include acquiring information on wave height
and frequency, which information is acquired by suitable means, such as
wave calibration and/or pressure sensors and/or radar and/or laser or
similar means, which means are preferably arranged along the sides of the
vessel to provide information on wave height and frequency.

[0040]Wave calibration is based on level tubes, which preferably are
arranged in the vertical direction along the vessel side. The reference
point for the lower part of the level tubes is the horizontal trim of the
vessel. By arranging a level sensor in each tube, the wave height at the
reference point can be read out at each tube. To indicate a wave
direction movement by this principle, a minimum of three sensors must be
used. Provided that at least three sensor tubes are arranged in each wave
frequency, it is possible to read out the wave direction. By calibrating
and synchronising the levels of each individual sensor between the
starboard and the port sides and the prow of the vessel, the actual
direction of the waves affecting the vessel can be obtained at all times.

[0041]This principle can also be used to calculate the changing fluid
volume/displacement which affects the movement of the hull in relation to
the vertical movement of the vessel, such as: LCB--longitudinal centre of
buoyancy, VCB--vertical centre of buoyancy and LCF--longitudinal centre
of floatation.

[0042]The system can also act as a passive stabilization of a vessel
provided with a system according to the invention. To perform passive
stabilization, the means for supplying and removing fluid to and from the
tanks include a controllable valve, arranged to each tank. As a vessel
travels against the tide and a controlled airflow has been calculated at
the top of the tanks, the tank(s) will be filled depending of the effects
of the sea. As the vessel then has its maximum filling of the tank(s), it
will have its maximum draught at that point. As the vessel starts to rise
due to the shape of the hull and buoyancy the airflow to the tank(s) will
close so that the vessel is weighted in such a manner that it is
prevented from rising. However, this weighting should disperse by the
time that the vessel reaches its uppermost position. This is achieved by
opening the airflow to the tank(s) and the fluid disappears immediately.

[0043]If we then look at a tank in the prow of the vessel, the ratio of
filling inside the prow should follow the level of the sea gets under the
effects of the waves, and thus the buoyancy in the prow is reduced as the
tank is filled with fluid. As the wave outside passes the rear end of the
prow, the wave will affect the hull for increased buoyancy, but as the
wave passes the prow, the fluid volume in the prow will reduce the wave
buoyancy on the hull behind the prow. As the vessel starts to lose
buoyancy due to the wave passing the prow of the vessel, it is desirable
for the fluid volume in the tank in the prow to be reduced, as a result
of which the vacuum in the tank in the prow is removed and the tank then
disperse the fluid which was used as counterweight as the wave passed the
prow. As the next wave hits the prow of the vessel, the tank in the prow
is again ready to be filled with fluid, so that the ratio of filling
starts to adapt to the actual wave height. One method for passive
stabilization according to the invention can be summarized in the
following steps: [0044]1. Acquiring information on the movements of the
vessel from a MRU and/or a VRU and/or a DP system or similar, which
provide information on the movements of the vessel, [0045]2. Acquiring
information on the status of the tanks in the system, [0046]3. Based on
information from the steps 1 and 2, calculating if the tanks should have
reduced or increased buoyancy, [0047]4. Opening the valve at the desire
of buoyancy and/or close the valve at the desire of increased buoyancy in
the tanks.

[0048]Steps 1 and/or 2, in addition to acquiring information on the
movements of the vessel, can preferably also include acquiring
information on wave height and frequency, which information is acquired
by means of suitable means, such as pressure sensors, radar and/or laser
or similar means, as wave calibration, which means preferably are
arranged along the sides of the vessel to provide information on wave
height and frequency.

[0049]By means of the system and the methods according to the invention,
the vessel can be provided with ballast and/or buoyancy according to what
is desired in relation to the coming changes in the environment, either
by passive or active stabilization of the vessel, or as a combination of
active and passive stabilization of the vessel, and in this way
compensate these changes, especially the vertical movements.

[0050]The system and methods according to the invention will be able to
work under different conditions, for example: [0051]1. Reduction of
pitch during sailing, which provides a fuel reducing effect, security
effect for vessel and persons and increased comfort for passengers and
sailors, [0052]2. Reduction of pitch and roll, which provides the same
advantages as under point 1, and secure working onboard or in conjunction
with another installation or vessel, [0053]3. Reduction of pitch, roll
and control of draught, which provides the same advantages as under point
1 and 2, and working with seabed installations, [0054]4. Control of
draught, which can be utilized during a difficult approach, or with
submersible vessels performing operations such as transport,
loading/unloading ships, which operate at quaysides, where low and high
tide levels can complicate loading/unloading.

[0055]The system according to the invention will not have any of the
above-mentioned problems encountered in the prior art systems, because
the tanks can work independently of each other, which results in a stable
system, with few possibilities for errors and dangerous situations, such
as instability or lack of capacity to provide ballast due to the limited
fluid volume. Stability can also be provided more rapidly compared with
existing systems, as traditional pumps will not be able to provide the
same capacity as the system according to the invention.

[0056]The system will further result in that vessels will be able to
withstand adverse weather and wave conditions, as the vessel can
compensate the effects of environmental changes, such as wave forces to a
greater extent than earlier. The total volume intended for active
stabilization can be used to increase the buoyancy of the vessel during
extreme wave and/or load conditions. Even though the vessel lies normally
low in the water in loaded conditions, this can be changed by using the
buoyancy volume it has available by not using the tanks with fluid. This
will result in reduced energy costs, as the vessel will be better able to
withstand the effects of the waves and thus be able to maintain its
position better than what is possible by only using propellers and
thrusters. In this way, the vessel will be able to reduce energy
consumption by a lesser use of thrusters and propellers.

[0057]Where a vessel is provided with a DP system, it receives signals
from satellites regarding on its actual position through antennae high
above the turning point of the vessel, and for the roll and pitch of the
vessel, this position will change by several metres in relation to the
vessel's actual position. If the vessel tilts over to the starboard side,
the position of the vessel will show a number of metres to starboard,
corresponding to the difference in length between the centre point of the
vessel's turning point and vertically up to the receiver antenna. The
propellers and/or thrusters will then try to prevent this change in
position and displace the vessel by the corresponding distance in metres
to the port side. If this movement occurs on a regular basis, the DP
system can compensate for it through its "learning function". The DP
system usually uses circa 20 minutes for each positioning to establish a
pattern for changes in wind, waves, current, etc. If the vessel is
provided with a system according to the invention, this margin of error
can be reduced considerably. Another advantage with the invention, which
does not receive much attention in the further description, is that the
system according to the invention has the possibility of varying the DP
learning pattern. In one situation active stabilization is used and the
DP learning system thinks that the waves, current and wind are according
to this, and in the next situation the system is turned off and the waves
appear different against the vessel. The DP system will thus be able to
more rapidly update changes by acquiring information from the different
sensors in the system in the present invention, so that rapid changes in
weather and/or operating conditions can be rapidly and precisely updated.

[0058]In addition to the above description, the present invention can
serve to change the draught of the vessel instead of vessels having to
operate on shallow water with always a too small draught.

[0059]NOx emissions can also be radically reduced with an active and
passive stabilization according to the invention. Where a vessel is
subject to movements, this is particularly affected by the diesel
engines, where changes of the diesel output constantly change the
handling of load, to which the vessel is exposed. The greater the changes
in the resistance in this activity, the poorer the combustion obtained in
a diesel engine. This can also be compared with the reduction of a
maximum speed of, for example, 15 to 14 knots, making the final sailing
distance covered at almost the same time, but at a significant economic
gain.

[0060]The present invention also ensures increased stability in comparison
to that of existing vessels.

[0061]From known accidents in shipping, it is known that the displacement
of ballast has not been carried out due to, for example, power failure.
If the vessels had been provided with a system according to the
invention, nothing would have affected the vessel in a power failure
situation, as the load in the stabilization tanks only would have flowed
out. If the system in addition was provided with an emergency backup
system, this could operate the valves to achieve stabilization even
though a power failure occurs.

[0062]Also cruise ships will benefit greatly from the invention, as they
can use the system to reduce pitch during sailing, which will result in
lower fuel consumption and better comfort for passengers with regard to
seasickness. This may also reduce the delay of the sailing and prevent
parts of the route from being shortened.

[0063]If the system is used on icebreakers, which have a specially formed
hull to break ice, which results in poorer sailing properties than for
common ships, this will ensure that icebreaker vessels are provided with
better stability conditions during sailing. Instead of having a large
amount of ballast water for pumping ballast from the stern to the prow of
a vessel, the vessel can have a normally designed stern, and take in sea
water at the stern and the prow I by means of vacuum instead of pumps.
Instead of transferring fluid from the stern to the prow of the vessel,
the vessel will still have the total ballast weight, but by taking in and
out weight directly from the sea, the weight will change rapidly. The
vessel can be relatively light at climbing on the ice, and rapidly
increase the weight if there are problems breaking the ice.

[0064]By means of the invention all vessels, where manual tasks are
carried out, would be able to achieve better stability, which results in
less vertical movement, which results in turn in better working
conditions and thus also fewer accidents.

[0065]By means of the invention there will be less need for heave
compensation of cranes and rigs, because the vessels will have less
vertical movement than that achieved with prior art technology, something
resulting in more rapid and precise operation at sea.

[0066]The above described examples show that the area of use is large and
the possibilities for a system according to the invention are many. In
the community today, where it is a huge focus on the environment, it will
be appreciated that all vessels using the present invention will save
fuel and thus have lower emissions.

[0067]It is obvious that all vessels must have the theoretical stability
requirements that apply today, and that the present invention applies in
addition to this.

[0068]It is further obvious that the system can be manual and/or
automatic, and that there will be possibilities of setting the trim as
desired. In some cases it will be sufficient for the vessel to have only
filled stability tanks to increase the total weight of the vessel. If the
vessel is not sailing, this can be sufficient for some tasks. Draught and
weight of the vessel can be adjusted to the most profitable operating
situation for each situation, and can rapidly be changed. In a sailing
situation today, it often happens that the vessels take on extra ballast
during bad sailing weather, but even if the weather improves, the sailing
continues with the same ballast as during bad weather.

[0069]Further details will appear from the following description.

EXAMPLE

[0070]The invention will in the following be described in detail with
references to the drawings, where:

[0071]FIGS. 1a and b show an example of a vessel in one state, seen in
cross-section from the side and above, respectively,

[0072]FIGS. 2a and b show the vessel in FIGS. 1a and 1b in another state,

[0073]FIG. 3 is a cross-sectional view of the vessel in FIGS. 1a-b and
2a-b, through a middle section of the vessel in FIGS. 1a-b and 2a-b, in a
third state,

[0074]FIGS. 4a-b show a vessel provided with a sensor means at the vessel
side,

[0075]FIGS. 5a and b show an example of how the system can utilize a
separate wave, and

[0076]FIGS. 6a and b show an example of the use of a fixed propeller in
the opening of the tank.

[0077]FIGS. 1a and 1b show an example of a vessel 10, where the system
according to the invention is arranged. The system includes, for example,
four tanks 11a-d, which tanks are arranged at suitable locations in the
vessel 10, where, as an example, one tank 11a is arranged in the front of
the vessel 10, two tanks 11b and 11c are arranged at each side, near the
middle of the vessel 10, and one tank lid is arranged at the rear of the
vessel 10. In this way the vessel, by means of the tanks 11a-d, will be
able to counteract the effects of the environment, such as waves hitting
the vessel alongside or abeam, or combinations of this.

[0078]Each tank 11a-d is adapted to the actual vessel 10, as regards size
(volume), shape and height above the fluid level in which the vessel is
floating, such as the sea level, which tanks are provided with openings
12a-d at the bottom. The openings 12a-d are large enough for a sufficient
volume of fluid to pass without cavitation or other resistance in the
openings of the tanks.

[0079]There will be a limitation of approximately 8 metres height of the
fluid in the tanks 11a-d, due to the physical laws for vacuum in fluids,
and for preventing the vacuum from evaporating the fluid instead of
providing elevation. The higher the vacuum that will be necessary in the
tanks, the less favourable it will be as regards economics/energy. The
larger the surface the tanks 11a-d have, the less need for energy is
required to achieve a high filling. As regards a vessel, a tank in the
front of the vessel will in any case be higher than a tank in the middle
of the vessel, this is because when sailing the waves affect changes at
the front of the vessel more than in the middle of the vessel.

[0080]The location of the tanks 11a-d will be dependent of which vessel 10
it is, and the properties which are desired for the vessel 10. The tanks
11a-d, which are to be operated to avoid pitch and roll, are most
effective the further out in the outer points of the hull they are
arranged, while the tanks 11a-d which are to be operated to control the
draught of the vessel, are most favourably arranged in the centre of the
vessel 10.

[0081]The further down in the vessel the openings 12a-d are arranged, the
more stable will be the control of the vacuum/pressure in the tanks
11a-d.

[0082]Further, the tanks 11a-d are provided with means 13a-d to control
the volume of fluid in the tanks, which means 13a-d preferably are vacuum
compressors or similar, which means 13a-d are used to control the
pressure/vacuum of the fluid surface, and in this way to lower or elevate
the fluid level to provide buoyancy, respectively ballast, in the tanks
11a-d for the vessel in different positions. The means 13a-d are
preferably arranged outside the tanks 11a-d, for easy maintenance. The
tanks 11a-d may also be emptied of fluid by supplying atmospheric
pressure to the upper part of the tanks 11a-d, if the situation so
permits and in this way there is no need for input power to empty the
tanks 11a-d.

[0083]To control the system and to provide information on the state of the
tanks 11a-d, the tanks 11a-d are further provided with measuring means
(not shown), such as pressure sensors/meters, floats, pressure pulses or
similar to provide information on the status of the tanks 11a-d to a
control system.

[0084]The system further includes, as mentioned, a control system, which
is provided with software/algorithms and/or programmed for controlling
the means 13a-d for controlling the fluid level in the tanks 11a-d, in
relation to the future movements of the vessel 10, especially the
vertical movement, which can be divided into roll, pitch and draught.

[0085]The control system will receive information from the means informing
on the state in the tanks at any time, and information on the movements
of the vessel. Information on the movements of the vessel can, in one
embodiment, be provided from a MRU (Motion Reference Unit) and a VRU
(Vertical Reference Unit), preferably with gyro stabilization, or similar
means providing information on vertical movements of the vessel. If the
vessel is equipped with a DP system, the control system can be provided
with direct input from this.

[0086]In addition the vessel is preferably provided with sensor means 14
(see FIGS. 4a and 4b), such as pressure sensors, radar and/or laser
and/or wave calibration or similar means, which means 14 preferably are
arranged along the sides of the vessel to provide information on wave
height and frequency. In the example shown, the means 14 are in the form
of wave calibration. Wave calibration is based on level tubes, which
preferably are arranged vertically along the vessel side. The reference
point at the lower part of the level tubes is the horizontal trim of the
vessel. By arranging a level sensor in each tube, the wave height in can
be read out at this point at each tube. To indicate a wave direction
movement by this principle, a minimum of three sensors must be used.
Provided that there is a minimum of three sensor tubes in each wave
frequency, it will be possible to read off the wave direction. By
calibrating and synchronizing the level of each sensor between the
starboard side, the port side and the front of the vessel, the actual
direction of the wave affecting the vessel at any time can be determined
at any time. This principle can also be used to calculate the changing
fluid volume/displacement which affects the hull movement in relation to
the vertical movements of the vessel, such as: LCB--longitudinal centre
of buoyancy, VCB--vertical centre of buoyancy and LCF--longitudinal
centre of floatation.

[0087]In this way the control system can be provided with information to
provide a picture of wave frequency, the direction of the wave and the
total changing buoyancy produced by the wave. The information provides
opportunities to predict the influence of the wave before the vessel
starts to respond.

[0088]The information from the sensor means 14 are preferably monitored by
a separate unit 15, which arranges the information for the control means.

[0089]The control system processes the information received and then
calculates the settings for the means 13a-d, which then sets the right
pressure and/or vacuum in the actual tanks 11a-d.

[0090]A vessel 10 provided with a system according to the invention will
be better able to counteract the influence of the environment around the
vessel, such as waves and other external factors affecting the vessel.
The vessel will also be better able to maintain its position than purely
by the use of propellers and thrusters, which are common for present
vessels. It will also result in reduced energy costs, as a system like
this requires fewer resources than for the use of thrusters and
propellers, as the vessel, to a lesser extent, will be affected by the
environment around the vessel, such as waves. It is, for example, for
offshore vessels provided with a DP system, the DP system which maintains
the vessel in position, while the system according to the invention
counteracts the effects from the environment on the vessel, such as the
effects of waves, which mainly are related to vertical movements.

[0091]FIGS. 1a-b illustrate an example of how a wave hits a vessel 10
lying in position, alongside in the bow with a force F. E.g., the vessel
lies in position in relation to another vessel or another offshore
installation (not shown). From, for example, the calculations of the DP
system of the vessel movements or information from a MRU and a VRU, and
information from measuring means in the tanks and sensor means along the
vessel sides, the control system according to the invention calculates
the ratio of filling in the different tanks 11a-d, which is necessary for
the vessel to be affected as little as possible by this wave. This
results, in this example, in that the control system, on the basis of
given parameters, sends control signals to the means 13a-d about the
ratio of filling for the different tanks 11a-d. To withstand the buoyancy
provided by the wave, the tanks 11a-c are, for example, filled 100%,
while the tank 11d, at the stern end of the vessel 10, will not be
affected to the same extent of the wave and is only filled to 10%. The
system can thus provide the necessary ballast in the front of the vessel
to maintain the vessel 10 in a vertical position, i.e., for example,
maintaining the same direction, the same distance to the seabed or the
same distance in relation to the offshore installation. As an
illustrating example, we can consider that a vessel 10 must have a tank
11a in the front of the vessel containing 200 m3 ballast to
compensate for the changes in the buoyancy in the front of the vessel
with waves of 3 metres, as illustrated in FIGS. 1a-b.

[0092]If the wave frequency in a given example is 10 seconds, this will
result in that the tank 11a, for example, must be filled with 200 m3
in 10 seconds, which results in that the fluid level in the tank 11a, for
example, must be elevated by 4 metres in relation to the fluid level 100
in which the vessel is floating, i.e. the sea level. This can according
to the invention be performed rapidly by using a vacuum compressor 13a
arranged in connection with the tank 11a, as described above. The vacuum
compressor 13a provides a negative pressure at the upper part of the tank
11, resulting in fluid being sucked in through the openings 12a into the
tank 11a to balance the pressure.

[0093]A vacuum compressor which, for example, is operated by a 200 kW
motor will be able to do this. By way of comparison, a traditional sea
water pump, such as an Anti heeling pump, will need a capacity of ca. 72
000 m3/hour to supply the same volume. To operate such a pump, a
motor of ca. 3850 kW would be required. This shows that large savings in
energy consumption can be made here, and that it will not be possible to
achieve a similar system as the invention by the use of prior art
technology. In addition there are also problems with pumps which are to
operate in sea water, as there could be corrosion problems for pumps, as
sea water is a corrosive medium, and water must be continuously pumped in
or out of the tank which must in this case be closed at the bottom. It
also means that this fluid volume reduces the load-carrying ability of
the vessel.

[0094]Open ballast tanks will, by definition, also reduce the total dead
weight, provided that there is not a valve at the bottom of the tank
which can be closed. Many considerations, on the other hand, show that it
would be favourable to provide the tanks with means for closing the
opening at the bottom. Even though a ballast tank, which is open at the
bottom, and with double securing at the top of the tanks to prevent the
air in the tank from escaping will theoretically maintain the buoyancy as
if the tank had a valve at the bottom. By incorporating a means for
closing, such as a valve or similar, at the bottom of the tank, it will
be possible, when not using active stabilization, to close the valve and
use the vessel as usual. Even though, by experience, it is known that
such a valve will leak, a stop valve can be used on the compressor tube
which is connected to the tank. The air will then be held in the top of
the tank so that the water only can compress the air in the tank, and the
buoyancy will be the same as if the tank was closed in the bottom. (By
incorporating air tubes into all bottom tanks on existing vessels, this
can contribute to preventing vessels which sail in shallow water from
becoming grounded and damaging the ballast tanks.)

[0095]As the wave in the example passes along the vessel, the need to
change the buoyancy/ballast in the different tanks 11a-d to counteract
the influence of the wave changes. FIGS. 2a and 2b illustrate a situation
in which the top of the wave is passing the stern end of the vessel. From
the calculations of the DP system of the future movements of the vessel,
and/or information from a MRU and a VRU, and information from the
measuring means in the tanks and sensor means arranged along the vessel
sides, the control system according to the invention calculates the ratio
of filling in the different tanks 11a-d which is necessary for the vessel
to be affected as little as possible by the wave, in the situation
described. The result of this is that the control system based on given
parameters send control signals to the means 13a-d about the ratio of
filling of the tanks 11a-d. As the vessel 10 here is affected the most by
the wave at the stern end of the vessel, tank 11d in the stern end of the
vessel is filled 100%, while the tanks 11b-c near the middle of the
vessel are filled with 75% and the tank 11a in the front of the vessel is
filled 10%. In his way the system according to the invention can
counteract the forces from the wave affecting the vessel, and maintain
the vessel 10 in a stable vertical position, i.e. maintaining the same
direction, the same distance from the seabed and maintaining the same
distance in relation to the offshore installation. If the tank 11d has
the same parameters as where used for tank 11a, the same calculations as
for tank 11a will provide the same result for tank 11d. Similar
calculations may as well be performed for the two tanks near to the
middle of the vessel.

[0096]As the tanks 11a-c here shall reduce their fluid volume in relation
to the situation in FIG. 1a-b, pressure must be supplied above the fluid
surface in the tanks 11a-c. If the openings 12a-c in the tanks 11a-c are
large enough to empty the tanks within 10 seconds, as was the wave
frequency in the example above, atmospheric pressure can be used. In this
way no power will be needed to empty the tanks. In this way, the power
consumption in the given example will only be the half of the power
consumption of the vacuum compressor within a period for the tanks 11a
and 11d, while it will be substantially less for the tanks 11b and 11c,
in a given period where the vessel lies in position in relation to a
offshore installation with uniform environmental conditions. If there is
need for changes which resulting in a need for buoyancy in one of the
tanks, the vacuum compressor can add extra pressure in the tanks and thus
contribute to increased buoyancy in the tanks. As mentioned above the
tanks can be provided with means for closing the openings of the tanks if
required.

[0097]Referring now to FIG. 3, this is a cross-section through the middle
section and the middle of tanks 11b and 11c of a vessel provided with a
system according to the invention. In this case, illustrated is an
example which shows a wave hitting the vessel 10 abeam with a force F.
The system according to the invention will here fill the tank 11b, which
lies closest to the strike side of the wave, entirely, providing the
vessel 10 with ballast on port side and thus counteracting the forces
from the wave and preventing tilting. In this way the vessel maintains an
approximately horizontal position. As the wave passes over to the
starboard side and provides total buoyancy on the hull, the total ratio
of filling for tank 11b and 11c must be changed, and tank 11c must thus
be filled and tank 11b emptied to counteract the forces from the wave.

[0098]FIGS. 5a and b illustrate that the system according to the invention
is energy saving. The system according to the invention can utilize a
separate wave striking, for example, tank 11a, as shown in FIGS. 5a-b.
The vacuum compressor 13a or an exhaust valve 13a can make the tank 11a
without pressure at entering the wave and the fluid flows freely into the
tank 11a. The tank 11a thus results in no buoyancy due to the wave
striking the first area of the vessel, while the height of the wave will
determine the ratio of filling of fluid in the tank 11a. As the wave
continues further to the back, towards the hull, the wave will affect the
buoyancy of the vessel. The vacuum compressor 13a then receives a signal
to increase the vacuum in the tank 11a, which thereby provides the tank
11a with the desired fluid weight to reduce the buoyancy of the passing
wave. This is illustrated in FIG. 5b, which shows the tank 11a being
gradually filled with ballast due to the wave (grey scale) and further
ballast supplied by the vacuum compressor 13a is shown as shaded area in
the tank 11a.

[0099]A method for active stabilization of a vessel by utilizing a system
according to the invention will now be described in more detail.

[0100]A method for active stabilization of a vessel includes the following
steps: [0101]1. Acquiring information on the movements of the vessel from
a MRU (Motion Reference Unit) and/or a VRU (Vertical Reference Unit)
and/or a DP system or similar, [0102]2. Acquiring information on the
state of the tanks of the system, [0103]3. Based on information from the
steps 1 and 2, calculating the ratio of filling for the different tanks
by means of a control system according to the invention, i.e. if vacuum
or pressure is to be supplied, where pressure only is supplied if the
level of the tank is to be lower than the fluid level in which the vessel
is floating, [0104]4. Providing means for controlling vacuum and pressure
in the tanks with settings based on the calculation in step 3, [0105]5.
Supplying pressure or vacuum to the tanks by means of means for
controlling the fluid volume in the tanks, until means for information on
the status of the tanks responds to the control system according to the
invention that the desired pressure or vacuum is achieved, [0106]6.
Repeating the steps 1-5.

[0107]Step 1 includes acquiring information from a MRU (Motion Reference
Unit) and a VRU (Vertical Reference Unit), a DP system or similar, which
information includes information on the movements of the vessel, and/or
information on wave height and frequency by means of suitable means, such
as wave calibration. By means of this information, the vessel can be
controlled to counteract these expected changes. A DP system is as
mentioned mainly incorporated for controlling the propellers and
thrusters of the vessel, but by means of the system according to the
invention, the information on the movements of the vessel can be used for
active and passive stabilization of the vessel, by supplying ballast or
buoyancy to the vessel through adapted tanks arranged at adapted
locations. This will provide entirely new possibilities for controlling
the vessel.

[0108]As today there exist laws and rules for wind and sea movements,
which set boundaries for when it is acceptable to carry out work on a
vessel together with other vessels/installations, the invention will
result in the vertical movements of the vessel being less affected by
waves and wind, and that the vessels being able to work during poorer
conditions and still be inside the statutory boundaries regarding waves
and wind, which means that vessels would have less time to wait for
calmer weather, before continuing with the work at hand.

[0109]Landing helicopters can also have an increasing movement problem,
and the present invention can make a significant contribution to solving
this problem.

[0110]Step 2 includes acquiring information on the state of the tanks of
the system, which is a premise for the control system according to the
invention to know if pressure or vacuum is to be supplied to the tanks.

[0111]The steps 1 and/or 2 can, in addition to acquiring information on
the movements of the vessel, also include acquiring information on wave
height and frequency, which information makes it possible for the control
system to form a picture of wave frequency, direction of the wave and the
total changing buoyancy provided by the wave. This is preferably
performed by means of sensor means, such as pressure sensors, radar
and/or laser and/or wave calibration or similar means, preferably
arranged along the sides of the vessel.

[0112]Step 3 includes the calculation of the ratio of filling in the tanks
based on the information acquired in steps 1-2, and predefined
parameters. The ratio of filling is controlled by supplying vacuum and/or
pressure in the tanks. If a tank is to be provided with ballast, the
system will calculate how much vacuum is needed to achieve the desired
ballast and thereby fill the tank with fluid. If a tank is to be provided
with buoyancy, the system will calculate how much pressure is needed for
supplying the tank to achieve the desired buoyancy.

[0113]The control system according to the invention will in advance be
provided with predefined parameters for the properties of the vessel and
the properties of the system. Different vessels will have different
properties, different tanks, different capacity for vacuum compressors,
etc., and the control system thus includes parameters so that the desired
behaviour and properties are achieved for the vessel. The control system
also includes security margins and other security instructions which have
to be followed if a critical situation occurs. The control system is also
provided with possibilities for manually changing the parameters, so that
the vessel can be provided with desired properties in relation to the
desired behaviour. The system can also be provided with special means for
critical situations, such as the tanks being provided with a throttle at
the top, which rapidly evacuates the vacuum in the tank and the fluid
will thus flow out. It will also in many conditions be relevant to have
an extra standby compressor for each tank, which will take over if
something should happen with the compressor.

[0114]The system can also be arranged so that, for example, if the draught
movement is critical for the vessel during an operation, the system will
be arranged to compensate additionally for this if a critical situation
occurs. This is similar to sailing in shallow waters as described above.

[0115]Steps 4 and 5 include providing the means for controlling vacuum and
pressure in the tanks with settings to achieve the desired ballast or
buoyancy in the tanks. Pressure or vacuum is supplied to the tanks until
means for information on the state in the tanks respond to the control
system that the desired vacuum or pressure is achieved.

[0116]Step 6 includes repeating the steps 1-5. As the situation of the
vessel and the environment continuously change, the system according to
the invention must also continuously change, so that the vessel exhibits
the desired behaviour. The system according to the invention thus
provides a closed loop control, which is self correcting.

[0117]The system can also function as passive stabilization for a vessel
provided with a system according to the invention. When a vessel travels
into the tide, and a controlled airing at the top of the tanks has been
calculated, the tanks will be filled according to the height of the sea.
As the vessel then has the greatest filling in the tanks, it will have
the greatest draught at the point in question. As the vessel starts to
rise due to the shape of the hull and the buoyancy behind tank 11a, the
airflow to the tank is closed, so that the vessel is weighted in such a
way that it will be prevented from rising, but this weighting will be
dispersed by the time the vessel reaches its uppermost movement by
opening the airflow of the tank so that the fluid flows out immediately.
That is to say that use is made of both the static movement the vessel
gets due to the wave and variations of the level of the wave outside the
hull. This change between reducing the buoyancy and fluid flowing freely
into the tank, and in the next moment the free fluid which has flowed
into the tank is retained as ballast. In this way, the passive
stabilization will work in the same way as a shock absorber on a car. The
opening ratio of the airflow will naturally be controlled and automated
by the control system, so that the system finds the best opening ratio to
prevent excessive wear on the mechanical parts of the system.

[0118]When the system according to the invention is to be used as a
passive system, use can be made of the information which already is
present at active stabilization, to operate a valve at the top of the
tanks, instead of controlling a vacuum compressor. A closed valve
corresponds to maximum power of the compressor and an open valve
corresponds to minimum power of the compressor.

[0119]A method for passive stabilization according to the invention can be
summarized in the following steps: [0120]1. Acquiring information on the
movements of the vessel from a MRU and/or a VRU and/or a DP system or
similar, which provides information on the movements of the vessel,
[0121]2. Acquiring information on the state in the tanks of the system,
[0122]3. Based on information from the steps 1 and 2, calculating if the
tanks should have ballast or increased buoyancy, [0123]4. Providing means
for controlling the fluid amount in the tanks with settings to open as it
is required to reduce buoyancy in tanks and/or to close these as required
to increase buoyancy in the tanks, [0124]5. Continuously repeating the
steps 1-4.

[0125]The steps 1 and/or 2 can also here, in addition to acquiring
information on the movements of the vessel, preferably also include
acquiring information on wave height and frequency, which information is
acquired by suitable means, such as pressure sensors, radar and/or laser
and/or wave calibration or similar means, which means preferably are
arranged along the sides of the vessel to provide information on wave
height and frequency.

[0126]If the information is not present, the valve must be adjusted
manually for the best possible effect by trial and experience, in the
same way as is done in an anti rolling stabilization tank, which is
filled up according to experience and conditions. For an adjustable shock
absorber on a car, the nozzle opening changes size, and in the same
manner a valve can be adjusted for the best possible effect for the pitch
of the vessel.

Modifications

[0127]The tank according to the invention can have a different shape, size
and height, and must be adapted to each vessel. In addition each vessel
will have a desired behaviour and properties, which system according to
the invention must be adapted for the achievement of the desired
behaviour and properties.

[0128]Means for controlling the buoyancy and ballast in the tanks are
preferably vacuum compressors/pumps, but the tanks can also be filled by
using, for example, a horizontal side propeller arranged in the lower
part of the tank, which is the opening of the tank.

[0129]To use a horizontal side propeller at the bottom of the tank is
considered less favourable even though the supply capacity may be
possible: [0130]1. The propeller must operate under water, [0131]2. The
vessel must go to dock for operations, [0132]3. Possibilities for leakage
and contaminations, [0133]4. Greater maintenance costs, [0134]5.
Dependent on service crew for maintenance, [0135]6. Greater investment
costs, [0136]7. More expensive installation.

[0137]Referring now to FIGS. 6a-b, which show an example of this. A
horizontal propeller arranged in the openings 12a-d of the ballast tanks
11a-d (only shown for tank 11a), which propeller 20 can be a similar to
the side propeller principle with adjustable pitch of the propeller
blades. The propeller blades can be controlled for possible filling or
emptying the tanks 11a-d. The propeller blades can be formed so that if
they are operated to a zero condition, they close the opening of the
tank. A retractable Azimuth propeller 21 can also be used in a situation
as described above. With a retracted Azimuth propeller 21, which is not
used for manoeuvring operations, it can be tilted so that the propeller
nozzle becomes a joint with the tank opening in the bottom of tank 11a-d.
This can then be used for filling and emptying fluid from the tanks
11a-d. FIG. 6a shows a fixed propeller in the opening of the tank 11a,
while FIG. 6b shows a retractable Azimuth 21 in a lower position M for
manoeuvring use, and in a retracted position O for filling and emptying
the tank 11a.

[0138]At their openings, the tanks can be provided with means for closing
the tanks, for example, to provide buoyancy.

[0139]A vertical side propeller at the bottom of the tank can also be used
to close the tank by that it includes specially shaped propeller blades
and hub, which results in that if its pitch are operated in a special
zone, an entirely closed construction is achieved, almost as a valve.

[0140]It will also be possible to use a hydraulic valve for this purpose,
e.g. by designing a hydraulic valve shaped as a propeller.

[0141]A "Vross", which is a submersible propeller, can in standby mode
(open position), be arranged to cover exactly an opening in the bottom of
the stabilization tank, and in this way it can ensure the changing of the
fluid amount in the stabilization tank. This can replace the vacuum
compressors or be used in addition to the vacuum compressors.

[0142]The existing compressors in the present system can also be used to
secure all ballast tanks with air supply. In the event of possible
accidents, which result in damage in the hull or sides of the vessel, the
compressors can supply sufficient air to the damaged tank to maintain the
original buoyancy in the tank, so that the vessel is prevented from
tilting and possibly sinking. The damaged tank must be arranged with a
stop valve to the tanks conventional airflow.

[0143]Vessels provided with brine, mud and cement tanks can use these as
buffer tanks for vacuum and air pressure to prevent rapid changes of the
compressor load.

[0144]Vacuum compressors can also be used to transport cooling water from
sea chests and via the cooler of the vessel. In this way there is no need
for the use of traditional seawater pumps.

[0145]A vacuum compressor can be used instead of traditional drainage
pumps and oil/water separators.

[0146]A cylindrical tank which can withstand vacuum and pressure loads can
be connected to a vacuum compressor, which has pipe connections to the
bilge pumps of the vessel. At negative pressure, this can be used instead
of present drainage pumps. Under closed valves to the bilge pump, the
vacuum compressor will evaporate the water from the contaminated bilge
water and lead the pure water vapour out to the atmosphere. After the
removal of water from the contaminated bilge water, the vacuum in the
tank is reversed to an over-pressure and the valve is opened to empty the
tank into a sludge tank. In this way, by means of the present invention,
the oil/water separator which is extremely difficult to get to work
satisfactorily according to the new regulations for pumping bilge water
overboard, which is at maximum 5 ppm, can be removed.

[0147]It should be mentioned that the above latter modifications can only
be performed when the vacuum compressor has sufficient free capacity.